1. Field of Invention
The present invention relates to a fuel cell system, particularly to a system for reducing energy consumption of an air feeding system in a fuel cell system, which efficiently cools air to be fed to a fuel cell thereof.
2. Description of Related Art
In a fuel cell, fuel gas such as hydrogen gas and oxidizing gas containing oxygen electrochemically react to produce electric current, which is taken out of a pair of electrodes provided on both sides of an electrolyte membrane. Especially, Polymer Electrolyte Fuel Cell using a solid polymer electrolyte has drawn attention as a power source of an electrical vehicle due to low operating temperature and easy handling. A fuel cell vehicle carries a hydrogen storage device, such as a high-pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank, and a fuel cell to which hydrogen gas is supplied from the hydrogen storage device to react with air. Electric energy produced by the reaction is taken out of the fuel cell to drive a motor connected to driving wheels. The fuel cell vehicle is thus an ultimate clean vehicle, which discharges only water.
In the fuel cell system, pressurization of air to be supplied to the air electrode has the following advantages.    (1) Increase in the oxygen partial pressure in the air, which contributes to an increase in a generated voltage and the power generation efficiency of the fuel cell.    (2) Because of an increase in the total pressure of the air, less moisture needs to be added at a gas temperature for creating moist air having the same dew point.    (3) Because of an increase in the total pressure of the air, the dew point rises, and the relative humidity can be maintained high in the fuel cell.    (4) Because the volume flow rate of the air is reduced, the cross-sectional area of a gas flow path in the fuel cell can be made smaller as long as the pressure loss is in an allowable range.
However, the pressurization of air to be supplied has the following disadvantages.    (1) Increase in electric power consumed by a compressor for pressurization.    (2) Temperature rise in the compressed air, resulting in an increase in capacity and in running cost of a cooler for lowering the raised temperature of the compressed air to an allowable temperature for the fuel cell.
The Japanese Patent Laid-Open Publication No. 2002-56865 discloses a fuel cell system, in which the energy consumption of the compressor is reduced by providing the system with an expander for recovering energy from the discharged air of the fuel cell, and a gas-gas heat exchanger to lower the temperature of the compressed air by exchanging heat between outlet air of the compressor and outlet air of the expander, which is cooled by adiabatic expansion therein.